LeadTrigger.apxt

// Trigger to search Accounts and create a new Case if a Lead Company Name is being created that matches an existing Account name
trigger LeadTrigger on Lead (before insert) {
    
    Set<String> companyNames = new Set<String>(); 					// Will hold the set of Lead Company Names
    Map<String, ID> accountNamesToId = new Map<String, ID>();		// Will hold the set of Account Names mapped to the Account ID
    List<Case> newCases = new List<Case>(); 						// Will hold the new cases to create
    for(Lead l : Trigger.new){										// Look at each Lead individually
        companyNames.add(l.Company);								// Add the Lead Company to our set
    }   
    
    // Get all existing Accounts with the same names as the Leads being inserted
    List<Account> existingAccounts = [SELECT Name FROM Account WHERE Name IN :companyNames];
    for(Account a : existingAccounts) {								// Look at each Account individually
        accountNamesToId.put(a.Name, a.Id);							// Add all Account names to a set and map it to the Account ID
    }

    for(Lead l : Trigger.new) {										// Go through list of Leads again
        if(accountNamesToId.keySet().contains(l.Company)) {			// If the Lead Company name is in the list of existing Account names
            Case c = new Case();									// Create a new case
            c.AccountId = accountNamesToId.get(l.Company);			// Get the Account ID that corresponds to this Account Name
            c.Subject = 'Account already exists for ' + l.Company;	// Set the subject
            c.Status = 'New';										// Set the status
            newCases.add(c);										// Add our Case to the list of Cases to create
        }   
    }
    insert newCases;												// Insert our Cases into Salesforce
}